Radiation detection devices are used in a variety of industrial, scientific, military, and government applications. Exemplary scintillator detectors have scintillator crystals made of activated sodium iodide or cesium iodide, or other materials that are effective for detecting gamma rays.
Generally, the scintillator crystals are enclosed in casings or sleeves that include a window to permit radiation-induced scintillation light to pass out of the crystal package. The light passes to a light-sensing device such as a photomultiplier tube (PMT), and the photomultiplier tube converts the light photons emitted from the crystal into electrical pulses. The electrical pulses are shaped and digitized by associated electronics and may be registered as counts that are transmitted to analyzing equipment.
Scintillators are used for nuclear and X-ray radiation detection. In response to a pulse of ionizing radiation they create a light flash. That light flash is recorded and analyzed by suitable instrumentation. Modern instrumentation relies on these light flashes having a consistent pulse shape. Variable pulse shape reduces the instrument performance. Usually the energy of the ionizing radiation pulse is proportional to the sum total of the emitted scintillation light (its time integral). Some scintillators show strong self-absorption coupled with delayed emission. In that case, the proportionality breaks down. The effect increases with scintillator size. Bigger crystals are desirable as they measure and count ionizing radiation more efficiently. But the precision of the measurement degrades for bigger crystals.
Existing instruments convert the light pulse into a concomitant electronic pulse. Analog instruments will transform the electronic pulse to create a new pulse shape with a pulse-height that is proportional to the integral over the original pulse. Digital instruments will attempt to perform a direct integration of the electronic pulse without applying that transformation.
The performance of both types of instruments degrades when a scintillator shows strong self-absorption of its own scintillation light coupled with delayed re-emission.
Some scintillators shows strong self-absorption of their own scintillation light. In scintillators with delayed re-emission of the absorbed light, the pulse shape of the observed light pulse will depend on where in the crystal the radiation was absorbed.
Traditional analog or digital MCA's (multichannel analyzers) measure deposited energies using a fixed shaping time (analog MCA) or a fixed integration time (digital MCA). Both devices rely on the pulse shape being constant, except for statistical electronic noise fluctuations around the average pulse shape. If the pulse shape becomes position dependent, traditional MCAs will not measure energies as accurately as possible.
Some scintillation crystals and fluids exhibit a phenomenon where the light pulse shape depends on the type of absorbed radiation. The same can be achieved using composites of different scintillators packaged together (phoswich). This is often used to distinguish gamma-ray detections from neutron detection within the same scintillator. This disclosure anticipates at least scintillator detectors utilizing a phoswich from the class of a combination of ZnS(Ag) and a plastic scintillator, a combination of NaI(Tl) and CsI(Tl), a combination of NaI(Tl) and a plastic scintillator.
Distinguishing types of radiation by the pulse shape of the scintillation light is called pulse shape discrimination. Existing methods for pulse shape discrimination are crude. They may cut the light pulse into two or three segments and compare the integrals within each segment to decide on the radiation type. Using abrupt segment boundaries is suboptimal at best, and will fail for scintillators with significant self-absorption of their own scintillation light.
Recently developed scintillators combine pulse shape discrimination capability with the ability to accurately measure gamma-ray (or beta-ray) energies. In these materials, the method used to measure the deposited energy must adapt to the type of radiation detected. Since pulse shapes are different for each type of radiation (gamma-ray, beta-ray, neutron, alpha-particle, etc.) a different method is required for each type of radiation to yield the most accurate measure of the deposited energy in each case.
There is a need to make the unique capabilities of new scintillator materials accessible to mainstream applications for improved radiation detection, monitoring and measurement. For instance, a high-resolution gamma-ray detector based on SrI2(Eu) can separate Cs-137 radiation (at 662 keV) from naturally occurring Bi-214 (at 609 keV). This strongly enhances the minimum detectable activity in food, water or soil samples, for the usually man-made fission product Cs-137.
Advanced digital signal processing, as described here, can improve the accuracy of gamma-ray spectroscopy in scintillators with strong self-absorption of their own scintillation light followed by delayed re-emission.
It can improve the accuracy of measuring deposited energies by different types of radiation when the energy computation is tailored for each type of radiation.
This supports using larger, self-absorbing scintillator crystals than would ordinarily be possible.